Apparatus &amp; Process for Airheater without Quench in Carbon Black Production

ABSTRACT

In the production of carbon black by the furnace process, heated process air (or oxygen enriched air) and fuel are fired into the reactor resulting in a flame. Carbonaceous feed stock is injected into this high temperature flame (about 1925 C) containing excess oxygen. The resulting cracking reaction should be arrested quickly (around 1300 C) so that secondary reactions that will decrease the yield of carbon black, do not continue. This quick cooling is typically done by quenching with a water spray. 
     The present invention relates to a process air heater to be located immediately after the reaction stopping quench, (around 1300 C), eliminating additional spray to control downstream equipment, achieving a high heat flux per unit volume and resulting is a smaller process air preheater. Further cooling of the effluent to the carbon black collector, typically a bag filter is achieved by a combination of water spray and heat exchangers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process airheater, herein referred to as NQAPH (No Quench Air Pre Heater), located after the reaction quenching water spray and subject to high temperature of the reactor effluent (around 1300 C). The airheater (NQAPH) has higher heat transfer rates and results in a smaller airheater compared to the prior art (APH). Spray water and associated energy loss are minimized. (162 kg/h of spray water and 123 kW of energy per 1000 nm3/h of process air)

2. Description of the Prior Art

In the prior art, mostly water has been used to quench the reactor effluent to al⁻rest the cracking reaction. Additional water is sprayed to cool the effluent further to protect the downstream equipment, typically process air heater, (APH). Such water has to be treated to reduce the impurities content for carbon black product quality as well as trouble free operation Of downstream equipment (due to the deposition of the impurities on the heating surfaces). This water, an increasingly important commodity, is converted to superheated vapor in this cooling process and the water and energy are irrecoverably lost to the atmosphere.

The prior art process air heater, (APH) is typically a shell and tube counter flow unit with the hot reactor effluent, cooled adequately with additional water spray, flowing inside multiple tubes and the process air flowing over the tubes in multiple passes over the tubes.

PRESENT INVENTION

In the present invention, the reactor effluent, after the reaction stopping quench, enters at high temperature (typically around 1300 C) and high velocity (typically 80 m/s), the inside of the tubes of the shell and tube type process air heater (NQAPH) and flows vertically. In the present invention,

-   -   The total process air is split into three streams.     -   One stream, approximately 50% of the total air flow will enter         the airheater (NQAPH) at the hot end and flow over the tubes in         multiple passes co-current to the reactor effluent.     -   The second stream, approximately 40% of the total air flow will         enter the airheater (NQAPH) at the colder end, and flow over the         tubes in multiple passes counter-current to the reactor         effluent.     -   The third stream, approximately 10% of the total air flow will         flow through the double plate bottom tube sheet (to cool the         tube sheet plates) and flow out of this tube sheet through the         inside of one or more cooling air return pipes and join the         second stream of air at the colder end of the airheater (NQAPH).     -   This third stream picks up heat at the double plate bottom tube         sheet as well as through the internal return pipe(s). When this         warm stream mixes with the second stream of air, the combined         stream is warmer than the entering second stream and helps         reduce the fouling tendency, by making the tubes warmer.     -   The co-current and counter current streams pick up heat from the         hot tubes inside which the hot reactor effluent is flowing and         join together about the middle and exit the airheater (NQAPH).         The hot process air flows through the external hot air piping to         the reactor.

In the present invention, the heat transfer flux increases by about 34%, the length of the tubes is reduced by about 25%, making the process air heater 25% shorter and making the usually vertically oriented airheater, structurally more stable.

In the present invention, the water quench in the prior art to control the hot air temperature of the process air heater, is eliminated saving water and energy. This energy can be recovered downstream of the airheater for useful purposes, instead of being lost to the atmosphere.

In the present invention, unlike the prior art (APH) where the hot exiting air is in contact with the double plate bottom tube sheet, cold incoming air of stream 1 is in contact with the double plate bottom tube sheet, keeping the tube sheet plates cooler and therefore, stronger.

In the present invention, the tube to bottom tube sheet weld, (FIG. 2), being in contact with the cold incoming air of stream 1, is, therefore, stronger and minimizes the potential for cracking and failure.

In the present invention, the shell, due to full length internal and external insulation, operates at lower temperature than in the prior art (APH) where only part of the shell is internally and externally insulated. (FIG. 6 shows the shell and tube temperatures for the NQAPH and prior art APH)

In the present invention, due to the shorter length of the unit, as in (005) above, and lower temperature, as in (009) above, the shell thermally expands less than the shell in the prior art (APH). The differential expansion between the tube and shell is also lower than in the prior art (APH). (FIG. 7 shows the thermal expansions of the tube, shell and the differential between them for the NQAPH and prior art APH)

In one configuration, the shell of this airheater (NQAPH) is fully externally and internally insulated. The internal insulation is retained in place by means of metal pins and metallic liner. The process air flows over this metallic liner.

In another configuration, the shell is only externally insulated and is provided with an internal metallic liner. The second stream of air will enter the airheater at the hot end and flows to the colder end through the annulus between the shell and the inner metallic liner. Typically turbulators are provided in the annulus to improve heat transfer and structural stability of the inner liner.

In one configuration, the internal baffles to create multiple passes for the airheater (NQAPH) are of the segmented type.

In another configuration, the internal baffles to create multiple passes for the airheater (NQAPH) are of the disc and donut type or any other method to create multiple passes of the process air.

In one configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of packing seals, with the tubes free to slide inside the seals.

In another configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of metallic bellow type seals welded to the sleeves in the top tube sheet or by any other method of sealing the process air from mixing with the hot reactor effluent, allowing for the hot tubes to thermally expand freely.

The process air for the CB production may be ambient air, Oxygen enriched air or 100% Oxygen.

DESCRIPTION

The invention is pictorially depicted in FIG. 1:

A is the entry of the Carbon black furnace effluent into the apparatus.

B is the exit of the effluent after being cooled in the apparatus

1 is the first stream of process air, entering at the hot end of the air heater through nozzle 12.

2 is the second stream of process air, entering at the colder end of the air heater through nozzle 13.

3 is the third stream of process air, entering from the cooling air header 7 into the double plate bottom tube sheet, 4, entering through cooling air nozzles 15. The bottom plate of this tube sheet 4 is refractory lined to protect this plate from the high temperature of the reactor effluent.

5 are the multiple tubes of the airheater inside which the hot reactor effluent flows at high velocity.

These tubes are connected to the bottom tube sheet 4 and the top tube sheet 6. This top tube sheet 6 is generally refractory lined

8 is the metallic shell of the airheater, fitted with internal insulation 10, retained by metallic pins and metallic plate 9. External insulation on the shell is shown by 11.

14 is the nozzle through which the heated process air stream leaves the airheater.

16 is the return pipe for the cooling air from the top plate of the bottom tube sheet 4 to the colder section of the airheater. This can be a single large pipe or multiple smaller pipes.

17 are the set of baffles inside the airheater which makes the process air to flow in multiple passes over the outside of the tubes 5. These baffles may of the segmented type (shown) or the disc and donut type.

FIG. 2 shows the tubes 5 are welded to sleeves 18 which is welded to the bottom tube sheet 4 and shows the tube to sleeve weld.

The tubes 5 may be connected to the top tube sheet 6 with packing seals 21 (FIG. 3), bellows type seals 22 (FIGS. 4&5) or any other suitable method of sealing the process air from mixing with the hot combustible reactor effluent.

FIG. 3 shows the packing seal type of joint. The packing seal 21 is provided with grooves and ceramic packing to let the tube freely expand inside the packing seal, at the same time making a difficult path for the air inside the shell to leak into the hot smoke. The tube end is machined to close tolerance with a very small clearance between the tube and the seal. When the hotter tube expands in diameter more than the colder seal, the clearance becomes smaller and the air to smoke seal tighter. A threaded follower 22 keeps the ceramic packing tight as a secondary sealing.

FIG. 4 shows a replaceable bellows type seal joint 23, with bellows in compression during service. This seal has an inner tube 24 (welded to the tube 5), outer specially shaped tube 25, multi convolution metallic bellows 26, connected by welding to 24 and 25. Ceramic packing made out of ceramic rope 27 is placed tightly between 24 and 25 to minimize the ingress of carbon black in the smoke getting to the inside of the bellows. Such ingress over a period of time will pack the bellows with hardened carbon black and make the bellows in effective, as the bellows squeeze together during service. The ceramic rope is kept in place by the retainer ring 28. The entire joint is connected to the top tube sheet 6 by welding to the tube sheet sleeve 29. The thin walled bellows are protected from the turbulence of the flowing process air by the bellows cover 30.

FIG. 5 also shows a replaceable bellows type seal joint 31, but it is designed for the bellows to be in tension in service. This type of joint is less sensitive to carbon black packing inside the bellows. When in service, the bellows stretch and the joint is still effective. This seal has an outer tube 24 (welded to the tube 5), inner specially shaped tube 25, multi convolution metallic bellows 26, connected by welding to 24 and 25. Ceramic packing made out of ceramic rope 27 is placed tightly between 24 and 25 to minimize the ingress of carbon black in the smoke getting to the inside of the bellows. The ceramic rope is kept in place by the retainer ring 28. The entire joint is connected to the top tube sheet 6 by welding to the tube sheet sleeve 29. No separate bellows cover is needed as the outer tube 24 protects the thin walled bellows from the turbulence of the flowing process air.

Constant load hangers 19 or counter weights are provided to keep the shell always in tension and prevent shell buckling under adverse operating conditions.

Sway brackets 20 are provided on the shell of the airheater to minimize the lateral movement of the airheater due to wind loads, uneven heating of the tubes 5 or any other causes.

FIG. 6 shows the profile of the tube wall temperature and the shell temperature for the present invention (NQAPH) and the prior art air heater (APH). It should be noted that the maximum tube wall and shell temperatures occur near the air outlet near the middle of the airheater (NQAPH). At the smoke entry and exit points of the present invention, (NQAPH), where the tubes are connected to the bottom and top tube sheets respectively, the temperatures are lower, making the joints stronger and less susceptible to failure.

FIG. 7 shows the thermal expansion of the tube and shell for the present invention (NQAPH) and the prior art airheater (APH). It should be noted that the thermal expansions are smaller for the present invention than the prior art airheater. The differential expansion between the tube and shell is also smaller for the present invention, resulting in smaller number of convolutions for the bellows type seal depicted in FIGS. 4 and 5. The lower thermal expansions are due to the shorter length of the tubes in the present invention (NQAPH) as shown in Table 1.

EXAMPLE

In a carbon black furnace, 17,000 nm3/h of hot air at 920 C is admitted along with adequate fuel (oil or natural gas) to raise the flame to a temperature around 1925 C. Hot carbonaceous feed stock is sprayed into this excess Oxygen rich hot flame and the ensuing chemical reactions convert the feed stock to carbon black and other gases. The reaction is stopped by a water spray at around 1300 C, which results in 29,130 nm3/h of gases and solid carbon black. In the prior art, this temperature is too high for the downstream airheater, (Reactor Air Preheater, APH), the gases are further cooled down by additional water spray of 2,750 kg/h to cool the gases to 1,050 C before entering this APH. The volume of gases entering the APH is 32,550 nm3/h. This water (2,750 kg/h) is not recovered and will be lost into the atmosphere. This additional water also causes problems in the downstream equipment like the bag filter with wetness.

With the proposed invention of airheater (NQAPH), all of this water is saved. A single airheater (NQAPH) will cool the gases from 1,300 C to 865 C. Further heat recovery from the CB containing reactor effluent down to the safe temperature for the Carbon Black collector can be achieved with additional heat exchangers for feed stock and fuel preheating, high and low pressure steam generation, tail gas preheating etc . . .

Table 1 compares the airheater (NQAPH) and the prior art airheater (APH), both of them designed to preheat the process air to 950 C.

TABLE 1 NQAPH APH Process Data: Heating side: Medium CB gases CB gases Volume of gases nm³/h 29,130 32,550 Temperature of gases entering C. 1,300 1,050 Temperature of gases leaving C. 865 640 Heat Transferred MM Kcal/h 5.126 5.126 Entering velocity inside tubes m/s 90.7 79.7 Exiting velocity inside tubes m/s 65.6 54.5 Additional Quench water kg/h 0 2,750 Cooling side: Medium Process Air Process Air Flow volume nm3/h 17,000 17,000 DBTS Cooling air flow nm3/h 1,700 1,700 Entering temperature C. 140 140 Exiting temperature C. 950 950 Heat Exchanger Data: Number of tubes — 108 108 Tube spacing mm 130 130 Length of each tube m 9.33 12.5 Total Number of baffles — 28 16 Heat flux_Co-current section kcal/m2 h 22,291 — Heat flux_Counter- kcal/m2 h 15,390 13,600 current section Overall Heat Flux kcal/m2 h 18,223 13,600 Total Heating Surface m2 281.3 376.9 Tube thermal Expansion mm 142 188 Shell thermal expansion mm 106 128 Differential thermal mm 36 60 Expansion 

We claim:
 1. An apparatus for heating up the process air (or Oxygen enriched air or oxygen) of a Carbon Black reactor by means of the hot effluent, located immediately after the reaction stopping quench without the need for additional cooling of the effluent to protect the apparatus. This apparatus will consist of multiple tubes through which the hot effluent flows and the process air flows in multiple passes over the outside of the tubes, transferring the heat of the hot effluent to the process air. The tubes will be connected to the single plate top tube sheet and the double plate bottom tube sheet, to keep the hot effluent and the process air from mixing with each other.
 2. The process air stream of the apparatus claimed in 1 will be divided into three streams, with stream one being about 50% of the total air, stream 2 about 40% of the total air and stream 3 balance of the total air (10%). Stream 1 will enter the apparatus in claim 1 at the hot end and flow over the tubes in multiple passes co-current to the hot effluent flowing inside the tubes. Stream 2 will enter the apparatus in claim 1 at the cold end and flow over the tubes in multiple passes, counter-current to the hot effluent flowing inside the tubes. Stream 3 will enter through multiple ports into the double plate bottom tube sheet of the apparatus claimed in 1, to keep the bottom tube sheet plates cooler and therefore stronger. The Stream 3 will then exit from the top plate of the bottom tube sheet and flow towards the cold end of the apparatus through a single large tube or multiple small tubes, co-current to the hot effluent. Stream 3 will then mix with Stream 2 and flow counter-current over the tubes in multiple passes.
 3. The cold air of Stream, flowing over the top plate of the double plate bottom tube sheet of the apparatus claimed in 1, keeps this plate cooler and no heat barrier like insulation is necessary on this plate.
 4. The cold air of Stream 1 and Stream 3 will keep the welds of the tubes to the double plate bottom tube sheet of the apparatus claimed in 1, cooler, reducing the potential for cracking of these welds that will result in catastrophic mixing of the hot effluent with process air stream.
 5. The flow of cold streams 1 and 3 over the double plate bottom tube sheet and the cold stream 2 over the top tube sheet will keep the bottom and top tube sheets at very nearly same temperature, keep the tubes and the tube holes aligned and minimize binding of tubes in the top tube sheet seals.
 6. The apparatus claimed in 1, operating at higher reactor effluent inlet and outlet temperatures, results in higher heat flux per unit volume, reduced heating surface and shorter in length.
 7. The apparatus claimed in 1, being internally and externally insulated along the entire length of the shell, and due to its shorter length, as claimed in 6, has the shell operating at lower temperature and thermally expanding less than in prior art.
 8. The apparatus claimed in 1, due to its shorter length, as claimed in 6, has the tubes expanding less than in prior art.
 9. The apparatus claimed in 1, per claims 7 and 8, has lower differential thermal expansion between the tube and the shell and results in lower relative movement of the tube at the tube to tube sheet seal.
 10. The replaceable bellows type seal joint 31, as shown in FIG. 5, designed for the bellows to be in tension in service. This type of joint is less sensitive to carbon black packing inside the bellows. When in service, the bellows stretch and the joint is effective. This seal has an outer tube (welded to the sleeve in the top tube sheet), a specially shaped inner tube (welded to the tube) and multi convolution metallic bellows, connected by welding to the inner and outer tubes of the joint. Ceramic packing made out of ceramic rope is placed tightly between inner and outer tubes of the joint to minimize the ingress of carbon black in the smoke getting to the inside of the bellows. The ceramic rope is kept in place by a retainer ring. The entire joint is connected to the top tube sheet by welding to the tube sheet sleeve and the tube. No separate bellows cover is needed as the outer tube protects the thin walled bellows from the turbulence of the flowing process air. 